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Tech Question

Terry Duris, Pataskala, OH, 1971 Dodge Challenger 440

A previous owner converted my ’71 to front discs using ’73parts. The car was not drivable when I bought it. I have replaced bothcalipers, rotors, pads and master from AutoZone. The problem is the pedalcan be pushed to the floor with little resistance. I am convinced there isno air as I have bled them several times even unbolting the calipers andfitting a block of wood in them and holding them at different positionswhile bleeding. ( I know the bleeder should point up). Could it be theproportioning valve? Should I try a master with a bigger bore? AutoZonecatalog shows a 1-1/32″ bore as correct. A 1-1/8″ was used on a ’71 Hemi. IfI pinch off the front hoses the pedal is hard.

Thanks, Terry

Terry-

Before I read totally through your letter, I was sure you had failed duringthe master cylinder “bench”-bleeding procedure (which is actually easierwith the MC bolted up.) If that happens, or the master is allowed to go dryduring the wheel-bleed procedure, you’re dead until you re-bleed the master.

Typically, I recommend blocking the ports on the master with SAE bolts(snugged, don’t go crazy w/torque) and confirming that the pedal is ROCKhard with almost zero travel. If not, the master is either airbound orsimply bad (internal leak). Your hose-pinch method seems to haveaccomplished the same thing, however, with so many reman master cylindersbad (I’d call the defect rate maybe 50%), I’d still like you to try that.You can use the bolts as bleeder screws. I’d be curious to hear what happenswith only ONE hose pinched — this is a good idea for learning if theproblem is only one side, or both.

Your test has, however, proved what I already guessed: Everything, and Imean everything, is good. A little explanation: Brake system valves cannotfail except to seize up and/or leak externally. Either way, the pedal willstill be firm. And master cylinder bore has no affect on trapped air.

Try this wheel bleeding procedure: First, yes, be sure the bleeder screwsare facing up. (I really hope the calipers are front hung and you don’t havethe taller ’73 steering knuckles, which triple the bump-steer, but that’sanother story). Connect a rubber hose, about 24-30″ long, sized to be a snugfit over the bleeder screw’s nipple. On the other end of the hose, find anut, preferably brass, that will just screw over the end of the hose. Dropthe hose/nut into a clean coke bottle (clear) that has at least an inch ortwo of clean brake fluid in it.

Bleed each wheel, starting from the farthest (RR) (you could probably skipthe rears, since you seem to have proven that they are OK), this way:

Having the car raised some helps. Using your right hand, pump the brakepedal to the floor, while resting your body on your left hand (on the floor)and looking at the Coke bottle. You’ll see bubbles. Hold the pedal at thefloor until you see the bubbles stop AND DISAPPEAR. Repeat as necessaryuntil there’s no more bubbles. Each time, only permit the pedal to come upSLOWLY to the returned position.

Fixed.

Rick

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